Seasonal Contributions to Recent Temperature Change (2001–2020) in De Bilt, the Netherlands

Observed near-surface air temperature in the Netherlands has increased markedly over recent decades, yet the seasonal drivers of this trend remain insufficiently quantified. This study analyses the relative contributions of individual seasons to the observed temperature trend at 2 m height in De Bilt, the Netherlands, over the period 2001–2020.

Using annual and seasonal temperature observations from the Royal Netherlands Meteorological Institute (KNMI), an energy and radiation balance was constructed. Surface temperature data were combined with satellite-derived measurements of radiation fluxes and cloud properties at the top of the atmosphere from the CERES mission. This approach enables attribution of temperature changes to variations in shortwave and longwave radiation, greenhouse effect strength, and non-radiative energy exchanges.

The analysis reveals a statistically robust warming trend of approximately 0.4 °C per decade over the study period. Seasonal attribution indicates that winter dominates the long-term temperature increase, accounting for approximately 72% (range 41–100%) of the total trend. Winter warming amounts to approximately 1.2 °C and is primarily driven by the greenhouse effect, contributing about 77% (46–100%) of the seasonal warming, supplemented by reduced reflected shortwave radiation and increased latent heat and convective fluxes.

The greenhouse effect, defined as the difference between upward longwave radiation from the surface and outgoing longwave radiation at the top of the atmosphere, shows strong seasonal variability. It exhibits a pronounced warming tendency in winter (6.5 W m⁻² decade⁻¹), while displaying a cooling tendency in spring (−2.9 W m⁻² decade⁻¹) and autumn (−3.4 W m⁻² decade⁻¹). This variability is mainly governed by changes in surface-emitted longwave radiation rather than changes in outgoing longwave radiation. The direct radiative contribution of CO₂ shows little seasonal variation, averaging 0.19 W m⁻² decade⁻¹ and accounting for less than 10% of the total greenhouse effect.

Overall, the results demonstrate that recent temperature change in De Bilt is strongly seasonally asymmetric, with a wintertime greenhouse effect fueled by changes in upward longwave radiation, playing a dominant role.